X-ray diffraction study of in segregation in MBE grown 
In_1-x Ga_x As/GaAs superlattices 

Philip Yashar, Manoj Pillai, Joanna Mirecki-Millunchick and 
Scott A. Barnett 

Department of Materials Science & Engineering Northwestern University,
Evanston, IL 60208 

X-ray diffraction (XRD) was used to study the composition modulation in 
In_1-x Ga_x As/GaAs superlattices grown by molecular beam epitaxy (MBE). 
Superlattice periods were 34.5, 57, and 93 A, with a nominal In_1-x 
Ga_xAs thickness of 10 A and a composition x = 0.5 in all cases.  The
experimental theta -2theta XRD patterns were simulated using a
kinematical calculation with a linear composition gradient at the
interfaces. Roughness was simulated by using random fluctuations in the
thickness of each layer and defects by random fluctuations in d-spacing. 
The In_1-x Ga_x As layers were assumed to be coherently strained to the
GaAs since the layer thicknesses were less than the critical value for
strain relaxation.  Simulations using a symmetric composition
modulation, as expected for interdiffusion, could not fit the
experimental data.  However, simulations using an asymmetric composition
modulation, expected due to In segregation during MBE growth, produced
good fits to the experimental data.  For this case where In segregation
was over distances greater than the In_1-x Ga_x As layer thickness, the
In content increased linearly with distance during In_1-x Ga_x As
growth, then decreased linearly after the In flux was stopped. Best fits
were obtained for a maximum In concentration of x = 0.2, with a gradual
decrease to pure Ga over ~ 35 A, and interfacial roughnesses of ~ 1A.